Method of pulling a semiconductor crystal from a melt



Aug. 23, 1966 RUMMEL ETAL 3,268,301

METHOD OF PULLING A SEMICONDUCTOR CRYSTAL FROM A MELT Filed D60- 2, 19632 Sheets-Sheet 1 Al lg. 23, 1966 RUMMEL ETAL METHOD OF PULLING ASEMICONDUCTOR'CRYSTAL FROM A MELT 2 Sheets-Sheet 2 Filed Dec. 2, 1963Fig.3

United States Patent 3,268,301 METHOD OF PULLXNG A SEMICONDUCTOR CRYSTALFROM A MELT Theodor Rummel, Munich, and Jiirg Dorner, Dachau,

Germany, assignors to Siemens & Halske Aktiengesellschaft, Berlin,Germany, a corporation of Germany Filed Dec. 2, 1963, Ser. No. 327,556Claims priority, application Germany, Dec. 3, 1962, S 82,693 6 Claims.(Cl. 23-301) Our invention relates to the method of producingsemiconductor crystals by pulling them from a molten mass of thesemiconductor material.

As a rule, such methods are performed by contacting a monocrystallineseed of the semi-conductor material with a melt heated to a temperaturea few degrees above the melting point and having the tip of the seedimmersed in the melt until an equilibrium between melt and crystal hascome about, whereafter the seed is pulled out of the melt at such aspeed that the semiconductor material adhering to the seed willcrystallize .onto the seed. According to a different method of thisgeneral type, the seed is pulled out of a melt while the latter is beingsuspended in a crucible-free or floating manner. To this end, the top ofa vertically mounted rod of semiconductor material is meltedby inductionheating and thereafter contacted by a monocrystalline seed. As soon asthe seed is wetted by the melt, the molten zone is caused to travel,starting from the seed and progressing in the direction of the rod axis.As a result, molten semiconductor material will freeze and crystallizeat the seed.

When performing such methods, it is often found that the crystals thuspulled leave much to be desired as regards the perfection required forelectronic semiconductor techniques and devices. Such faults as latticedislocations and tothe disturbances in crystalline structure areobserved and may greatly impair the subsequent fabrication or use of thecrystals needed for transistors, diodes and other semiconductor devices.

Essential for securing crystalline perfection of the pulled crystals arethe symmetry of the heating zone, as well as a certain after-heatingzone, and the shape of the solidifying front, that is, the shape of theisotherms in .the region of the still plastic portion adjacent to thesolidifying front in the just frozen material.

It has been proposed to better approach crystalline perfection bypassing the pulled crystal through a zone that reduces the radialtemperature gradient in the rod, and to thereafter pull the crystalthrough a zone that increases the axial temperature gradient.

According to another proposal, the shape and position of the isothermsin the pulled crystal are influenced by adjusting for a given roddiameter the pulling speed to a given value. This expedient, however,cannot be employed if the rod is to possess a given constant dopantconcentration over its entire length because in this case the pullingspeed is already fixed by the desired dopant concentration. I

It is an object of our invention to devise a possibility of improvingthe crysal perfection also in cases of the latter type, namelyregardless of whether or not the pulling speed is predetermined by otherrequirements or desiderata.

According to our invention, we perform the pulling of a semiconductorcrystal from a melt with the aid of a crystal seed by proceeding in thefollowing manner. During the pulling operation we pass an electricdirect current through the melt and the crystal being pulled and therebysubject the freezing region of the growing crystal to the so-calledPeltier effect dependent upon the currentflow direction and currentintensity. We have discovered 3,268,301 Patented August 23, 1966 icethat in this manner the lattice disturbances in the pulled crystal canbe considerably reduced. This method is applicable with melts heldfloating, that is, suspended without the use of a crucible, as well aswith melts located in a crucible. However, the invention is preferablyand most readily practiced with a crucible-free crystal-pulling ormonocrystal-forming zone-melting operation.

The passage of electric current produces heating or cooling by thePeltier effect not only at the boundary face between different contactmaterials but also at the boundary face between the solid and liquidphase of one and the same conducting material, particularlysemiconductor material. For example silicon and germanium are known toexhibit a positive Peltier effect; that is, heat is absorbed at theboundary face between the solid and liquid phases of these substances ifthe solid phase is conected to the positive pole and the liquid phase tothe negative pole of a direct-voltage source, whereas the reverse polingcauses the Peltier effect to produce heating at the other boundary face.Consequently, when a germanium crystal, for example, is subjected tozone melting with the melt held between two solid crystal pieces, and adirect current is passed between these two pieces, the boundary facebetween one of the crystal pieces and the melt is subjected to coolingwhen positive poling is applied, whereas with negative poling of thecrystal piece the other boundary face becomes heated.

When a crystal is being pulled out of a melt, a radial temperaturegradient develops in the pulled crystal, the solidifying front and theisotherms being curved in the crystal being pulled. The molten zone,produced in most cases either by an induction coil or by radiation fromthe outside, possesses a higher temperature in the marginal zone of thecrystal than in the center region, the solidification of the crystalcommences from the center region of the rod, and the solidifying frontis curved from the seed toward the melt. At some distance from thesolidifying front, the heat radiation of the crystal surfacepreponderates over the original temperature difference between themarginal zone and the center region, so that here the thermal conditionsare reversed; the isotherms are then curved toward the seed. In thepulled crystal, therefore, the isotherms, starting from the concavelycurved solidifying front, first become shallow and then, at somedistance from the front, convert to a convex curvature.

The radial temperature gradient in the crystalliz-ing semiconductormaterial as it is being pulled out of the melt is the larger, the morestrongly the isotherms are curved. A planar or weakly convex solidifyingfront, therefore, is particularly favorable if dislocations in theresulting crystal are to be prevented. This is because in this case theslightest thermal tensions occur in the immediate vicinity of thesolidifying front and hence in the region in which the semiconductormaterial is still plastic.

The shape of the solidifying front and of the isotherms can also bemodified by adjusting the pulling speed. For each crystal there can befound a pulling speed at which the most favorable shape of thesolidifying boundary face occurs; however, as mentioned, the pullingspeed in most cases is already fixed by the desired constant degree ofdoping. In this case the invention provides for a new controllingparameter. That is, the heating or cooling occurring as a result of thePeltier effect in dependence upon current intensity and currentdirection, can in any case, and consequently for any pulling speed andany desired crystal diameter as well as for any material, be so adjustedthat the solidifying front assumes the most favorable shape, and theoccurring Joules heat simultaneously reduces the occurring tensions. Ifit is desired to still further increase the resulting tension-reducingheating, it is in some cases of advantage to superimpose an alternatingcurrent upon the direct current. The alternating current does not resultin a Peltier effect, but it heats by Joules heat the semiconductormaterial, particularly the solid crystal piece. That is, by employing asuperimposed alternating current, the amount of Joules heat can be givena desired dosage independently of the Peltier heat.

The invention will be further described with reference to theaccompanying drawings in which FIG. 1 shows schematically a portion of asemiconductor rod which is being processed by crucible-free zone meltingwithout utilization of the Peltier effect;

FIG. 2 shows a corresponding portion of a semiconductor rod alsoprocessed by floating-zone melting but simultaneously subjected to thePeltier effect in accordance with the present invention; and

FIG. 3 shows schematically and in section a device for performing themethod of the invention.

As shown in FIG. 1, a molten zone 3 is located between an upper rodportion 1 and a lower rod portion 2, the zone 3 travelling in thedownward direction so that the upper rod 1 is grown in form of amonocrystal out of the melt. The direction of crystal growth isindicated by the arrow 4, this direction being identical with thedownward travel direction of the molten zone 3. Shown schematically inthe crystallizing rod portion 1 are a group of lines of which the onedenoted by 5 represents schematically the solidifying front, whereas theother lines indicate isotherms. The isotherms denoted by 6 have theshape most favorable for the solidifying front as well as for theisotherms adjacent to the solidifying front, if the resulting crystal isto be as free of dislocations as feasible.

By virtue of the invention, namely by subjecting the growing crystal aswell as the melt to the Peltier effect with the aid of direct current,the position of the solidifying front relative to the heating device canbe changed. That is, the solidifying f-ront can be displaced as desiredin the direction of the crystal axis, and this can be done in the sensepromoting the desired reduction in dislocation density. That is, byheating one of the solid-liquid boundary faces, for example thesolidifying front, with the aid of the Peltier effect, requiring, forexample for germanium or silicon, a negative poling of the solidifiedcrystal and hence a current-flow direction opposed to the zone-pullingdirection, the solidifying front becomes displaced in the direction ofthe crystal being pulled. By applying a given current intensity ordensity, readily ascertainable by pretesting for each given diameter andeach pulling speed, the solidifying front can thus be displaced up tothe isotherm face at which the smallest thermal tensions will occur.

For example, when producing a silicon monocrystal having a diameter ofabout 10.5 mm. from a floating melt at a pulling speed of approximately2.5 mm./minute, a current density of approximately 100 amps per cm. hasbeen found to be particularly favorable. Approximately the same currentdensity is preferably employed for somewhat different crystal diametersand somewhat different pulling speeds, for example diameters of to 11mm. and speeds of 2 to 3 mm. per minute. During the tests made, thecrystal was kept in rotation about its own axis at the rate of 60revolutions per minute. The smallest dislocation density is obtained byhaving the current flow in opposition to the pulling direction, that is,when the crystal piece being pulled out of the melt is connected to thenegative pole of a direct-voltage source. The other boundary face of themolten zone is subjected to cooling by Peltier effect, so that here,too, the relative position of the solid-liquid boundary face isdisplaced in the direction of the rod axis toward the heating zone.

The resulting conditions are schematically shown in FIG. 2. The crystal1 growing in the direction of the arrow 4 exhibits a particularly slightdislocation density. The isotherms 6 near the solidifying front 5, aswell as the solidifying front itself, are slightly convex toward "themelting zone 3, so that only slight thermal tensions occur in theportion of the crystallizing rod that is adjacent to the solidifyingfront and is still plastic. The direct current is passed through thecrystal in the direction of the arrow 7. For this purpose, the crystalpiece 1 crystallizing out of the melt is connected to the negative poleof a direct-voltage source. The boundary face is subjected to heating,and freezing takes place only at a larger distance from the heatingdevice 8. The conditions are reversed at the other boundary face. Thisboundary face is cooled by the Peltier effect so that the source rod 2is melted later than would otherwise be the case.

FIG. 3 shows a rod 1, 2 and a heater 8 according to FIG. 2 inconjunction with suitable processing apparatus. While the molten zone 3is being heated by the highfrequency induction coil 8, a direct currentis passed longitudinally through the rod from a source 9 of constantvoltage. The source 9 is connected through an inductance winding 10 anda current-control resistor 11 between terminals 12 and 13 conductivelyconnected with respective holders 14 and 15 of highly pure graphite inwhich the two ends of the rod are fastened by means of graphite clampingscrews 19. Simultaneously impressed across the terminals 12 and 13 is analternating voltage from a source 16 through a control resistor 17 and acapacitor 18.

During operation, the semiconductor crystal 1 grows in the direction ofthe arrow 4 as the heater coil 8 moves downwardly. The direct currentpasses through the rod in the upward direction indicated by an arrow 7.That is, the rod portion 1 recrystallizing out of the melt 3 isconnected to the negative pole of the direct-current source 9. Thesemiconductor rod and the holders 14, 15 are mounted in a tubularprocessing vessel 20 whose walls consist of quartz and which is providedat its respective ends with inlet and outlet ducts for protective gassuch as argon, the gas flow being indicated by arrows 21. Thecurrent-supply leads pass through respective seals 26 at the top andbottom of the vessel. The high-frequency coil 8 is fastened to avertically displ'aceable support 22 which passes through a glide seal tothe outside of the vessel. A rack 24 on support 22 meshes with a pinion25 which is slowly driven during operation for displacing the heatercoil in the direction along the semi-conductor rod. The rod portion 1crystallizing out of the melt can be kept in rotation by turning theupper holder 15 or its shaft from the outside of the vessel, this beingindicated by an arrow 27.

As an example of applicable operating data, the following are mentioned.A rod of phosphorus-doped silicon of 0.5 ohm-cm. specific resistance,having a diameter of 0.9 cm., can be processed with good resultsaccording to the invention by passing through the rod a direct currentof 47 amps/cm. density in the direction opposed to the zone-traveldirection, the rod piece crystallizing out of the melt being negativelypoled. Simultaneously superimposed upon the direct current is analternating current of 65 amps/cm. density. The zone-pulling speed is1.74 mm./min.

In the following comparative tests, the direct current passing throughthe crystal and the melt was changed, Whereas otherwise the testconditions were kept constant. Used in the tests were phosphorus-dopedsilicon rods of 10.5 mm. diameter. The zone melting was performed at azone travel speed of 2.5 mm./min. Only direct current of amps/cm.density was employed. It was observed that in cases where the directcurrent was passed through the crystal and melt in the direction opposedto the zone-pulling direction, the dislocation density in the pulledcrystal was about 5000/crn. Without applying the direct current, thedislocation density under otherwise the same test conditions was foundto be approximately 50,000/cm. When the current was passed through thecrystal and the molten zone in the zonepulling direction, thedislocation density was about 15,000/cm. under otherwise the same testconditions.

The results show that the Joules heat in conjunction with the coolingdue to the Peltier effect with a corresponding poling, already affords aconsiderable improvement toward better crystal perfection, but that afurther considerable approach to crystal perfection is achieved byreversing the polarity of the direct voltage and hence the current-flowdirection.

A simple way of determining the dislocation density in crystals is totreat the crystal for a short period of time with an etching agent, forexample a mixture of nitric acid and hydrofluoric acid in 1:1 ratio. Thefaults in the crystal lattice, mainly the lattice dislocation, thenbecome manifest in the form of etch patterns. This method was employedin the foregoing tests.

We claim:

1. The method of pulling 'a semiconductor crystal from a melt of thesemiconductor material with the aid of a crystal seed, which comprisespulling a seed crystal from a crucible-free melt zone passing during theentire pulling operation an electric direct current serially through thecrystal being pulled and then the melt in 'a direction opposite to thedirection of pull thereby developing a freezing region at thesolid-liquid interface of the seed crystal and melt due to a Peltiereffect whereby lattice faults in the pulled crystal are reduced due tosaid effect.

2. The method of pulling a semiconductor crystal from 'a melt of thesemiconductor material with the aid of a crystal seed, which comprisesholding the melt cruciblefree between a lower vertical supply rod of thesemiconductor material and an upper vertically suspended crystal,pulling a seed crystal from a crucible-free melt zone, passing duringthe entire pulling operation an electric direct current in seriesthrough the crystal and then the melt in a direction opposite to thedirection of pull thereby developing a freezing region at thesolidliquid interface of the seed crystal and melt due to a Peltiereffect whereby lattice faults in the pulled crystal are reduced due tosaid effect.

3. The method of pulling 'a semiconductor crystal from a melt of thesemiconductor material with the aid of a crystal seed, which comprisesholding the melt cruciblefree between a lower vertical supply rod of thesemiconductor material and an upper vertically suspended crystal,pulling a seed crystal from a crucible-free melt zone, and impressingbetween the crystal and the rod a direct voltage by connecting positivepotential to the rod and negative potential to the crystal to passduring the entire pulling operating a direct current serially throughthe crystal and then the melt in a direction opposite to the directionof pull thereby developing a freezing region at the solid-liquidinterface of the seed crystal and melt due to a Peltier effect wherebyreduction of lattice faults in the pulled crystal results.

4. The crystal pulling method according to claim 2, which comprisescontinuously turning the rod about its axis while passing the currentthrough melt and crystal during pulling of the crystal.

5. The method of pulling a semiconductor crystal from a melt of thesemiconductor material with the aid of a crystal seed, which comprisespulling a seed crystal from a crucible-free melt zone, passing duringthe entire pulling operation an electric direct current serially throughthe crystal being pulled and then the melt in a direction opposite tothe direction of pull thereby developing a freezing region at thesolid-liquid interface of the seed crystal and melt due to a Peltiereffect and superimposing an alternating current upon the direct current,whereby the dislocation density in the pulled crystal is reduced.

6. In the semiconductor crystal pulling method according to claim 2,said rod consisting of silicon and having a diameter of about 10 toabout 11 mm., the pulling speed being about 2 to about 3 mm. per minute,and said direct current having a density of approximately amps per cm.

References Cited by the Examiner UNITED STATES PATENTS 2,792,317 5/1957Davis. 2,932,562 4/1960 Pfann 23-301 2,937,216 5/ 1960 Fritts et a1. 623X 2,970,895 2/1961 Clark et al. 23-301 X 3,058,915 10/1962 Bennett 23273X 3,152,022 10/ 1964 Christensen et a1. 23273 X NORMAN YUDKOFF, PrimaryExaminer.

G. HINES, Examiner.

1. THE METHOD OF PULLING A SEMICONDUCTOR CRYSTAL FROM A MELT OF THESEMICONDUCTOR MATERIAL WITH THE AID OF A CRYSTAL SEED, WHICH COMPRISESPULLING A SEED CRYSTAL FROM A CRUICIBLE-FREE MELT ZONE PASSING DURINGTHE ENTIRE PULLING OPERATION AN ELECTRIC DIRECT CURRENT SERIALLY THROUGHTHE CRYSTAL BEING PULLED AND THEN THE MELT IN A DIRECTION OP-